At what point do you...

Personally, I always buy brand new as long as I can afford it. Will buy used if the item is no longer available new and if it’s a must have for me, but mostly only in person so I can verify its authenticity and functionality or will order if it’s an item that I already have and already know what to expect and to look for. If the savings is substantial (relative to each individual) I would be more inclined to buy used from a reputable source at half price or less and only if it can be returned without hassle. I don’t have a problem overpaying for something to get what I want the way I want it. Good service merits its own price tag.

At least with cables, since it sounds like that’s what you’re interested in there’s not much to go wrong there but would be beneficial to already have a one to compare to and make sure it’s authentic. Lots of fake stuff going around...

I would never buy open box or used speakers without auditioning.  Highly likely to have been abused or damaged and a hassle to repack, carry and return.

I always buy used unless no used available, even then I would wait a while, having hifisearch show me that item or category 'first shown, new first'. check every am, bang: strike like a viper.

I am confident in my skills to evaluate the seller and item and money movement, and so far have not been disappointed. a few things have gone wrong (primarily bad packaging), all resolved, or refunded.

Worried about seller's packing skills, or local pickup only: I use UPS Pack and Ship, seller simply drops unpacked item nearby in your name, walks away. You make your own arrangements with UPS, you pay UPS directly, No need to buy insurance: UPS is responsible for any damage which is unlikely.

If you really want something, cannot find an open box (which get maker's full warranty), or 'safe' used, gotta pay the price. Then I think: "will I regret it? will I regret not getting it?"